Combustion within the combustion chambers of an internal combustion engine is controlled by precise amounts of combustion air and fuel metered at a set ratio into the combustion chamber. The ratio of air and fuel in the mixture is conventionally known as the air-to-fuel ratio (AFR) and expressed as lambda, with stoichiometric combustion having a lambda value of 1. Regulatory standards limit the concentrations of noxious gases allowed in the products of combustion or exhaust gases emanating from the vehicle. Regulated emissions are typically hydrocarbons (HC), carbon monoxide (CO), and nitrous oxides (NO.sub.x). Vehicles are equipped with catalytic converters to transform the regulated or noxious emissions to harmless, gaseous emissions. The catalytic converter systems in use today typically include a three-way catalyst, a TWC catalyst, capable of converting CO, HC and NO.sub.x.
Conventional TWCs work efficiently at or near stoichiometric conditions. When the engine runs rich, there is insufficient oxygen for CO and HC combustion. If the engine runs excessively lean, NO.sub.x concentrations in the exhaust gases increase and make it more difficult for the TWC to cleanse the NO emissions. To address this problem, TWCs are constructed with a composition (i.e., ceria) that can store oxygen when the engine runs lean and release the oxygen when the engine runs rich ("OSC"). With this capability added to the TWC, typical engine control strategies cycle the AFR about stoichiometric to provide alternating rich/lean exhaust gas mixtures. The engine control strategy prevents excessive NO.sub.x emissions from being produced during its "lean" condition while the oxygen stored during the "rich" operation is used in the "rich" mode to combust HC, CO and H.sub.2.
As the catalytic converter ages, its ability to convert noxious emissions diminish. The prior art has recognized that concentrations of certain emissions within the products of combustion produced by the engine can be controlled by changing the operating conditions of the engine to adjust for the aging of the catalytic converter. In this manner the engine control strategy can insure, to some point, that the emissions from the vehicle continue to meet regulatory standards thus prolonging the life of the catalytic converter provided, of course, that the emissions can be accurately measured. Reference can be had to U.S. Pat. No. 5,609,025 to Abe and U.S. Pat. No. 5,557,929 to Saito et al., incorporated by reference herein, for techniques controlling the air-to-fuel ratio and reference can be had to U.S. Pat. No. 5,623,824 to Yukawa et al., incorporated by reference herein, for controlling the perturbation rate (frequency) as well as the air-to-fuel ratio. In addition, it is known that exhaust gas recirculation (EGR) can be controlled to limit NO.sub.x emissions. Reference can be had to U.S. Pat. No. 5,426,934 to Hunt et al.; U.S. Pat. No. 5,564,283 to Yano et al.; and U.S. Pat. No. 5,570,673 to Isobe, incorporated by reference herein, for various techniques controlling EGR as well as other vehicle operating conditions such as spark timing, mass air flow, etc. to produce products of combustion having certain emissions capable of being converted by the TWC.
Any engine control strategy can only be as good as the instruments used to measure the results achieved when the control strategy is implemented. Specifically, the engine control strategies discussed above are implemented with the intent to insure that the vehicle meets emission standards while maintaining vehicular driveability. If the emissions are accurately measured, then the engine control technique, any engine control technique, can be verified. Until this invention, there was no known, reliable, consistent and commercially feasible system which could be mounted in the vehicle for continuous monitoring of the emissions actually produced by the vehicle, let alone, function as a feedback control for regulating the engine operating condition.
Engine control strategies and catalytic monitoring systems in widespread use today sense the oxygen concentrations in the exhaust gases. The oxygen sensor readings are used to set the air-to fuel ratio and also to monitor the OSC of the TWC. Such systems are inherently defective for monitoring emissions because they are incapable of directly measuring the regulated emissions.
Current OBD methods based on OSC rely upon correlating the deterioration of OSC with the loss of EC efficiency. In fact, OSC of the TWC has been demonstrated to poorly correlate with hydrocarbon conversion efficiencies. See J. S. Hepburn and H. S. Gandhi, The Relationship Between Catalyst Hydrocarbon Conversion Efficiency and Oxygen Storage Capacity, SAE paper 920831, 1992 and G. B. Fischer, J. R. Theis, M. B. Casarella, and S. T. Mahan, The Role of Ceria in Automotive Exhaust Catalysis and OBD-2 Catalyst Monitoring, SAE paper 931034, 1993. As the allowable emission levels are reduced with increasingly more stringent regulations, the OSC correlation becomes highly unreliable. Specifically, existing techniques which compare AFR before and after a portion of the TWC (sometimes at known operating engine conditions) will not have the required sensitivity to meaningfully monitor the emissions.
Further, emission regulations now coming into effect require that the vehicle control the emissions when the engine is cold or on start-up and not only when the vehicle has reached its operating temperature. This requirement is met by the use of close-coupled, light/off catalytic converters, which may be void of rare-earth metal oxides, the major components of OSC. Such catalytic converters may lack significant oxygen storage function and cannot employ or use oxygen sensors to measure oxygen storage capacity. Thus, current dual EGO sensor methods are incapable of OBD of such close-coupled, light/off catalytic converters.
In addition to such fundamental considerations which strongly dictate that measuring OSC will not meet proposed regulated monitoring standards, there are other disadvantages to the current system. Currently, the catalytic converter design, itself, has to be structured to put OSC into the monolith at concentrations sufficient to allow the aforementioned correlation to take place and to continue to take place as the converter ages. Further, the addition of OSC promotes sintering of the catalytic converter at high temperatures. A more durable catalytic converter could be designed at less expense and with higher concentrations of catalysts to give better conversion efficiencies at a longer life if OSC was not present or present in minute concentrations.
Exhaust gas oxygen (EGO) sensors (usually either a heated EGO (HEGO) or a universal EGO (UEGO)) are typically placed upstream and downstream of the catalytic converter and primarily function for engine fueling control. They are also used to determine OSC of the TWC. Both sensor types possess a platinum based coating which combusts residual reductants and oxidants present in the gas stream. Only after substantially all residual gases are combusted, is the oxygen concentration remaining determined. For rich mixtures the sensor detects no residual oxygen. For lean mixtures the oxygen not consumed by the combustion on the platinum electrode surface is detected. Thus, it is fundamentally important to note that neither sensor can detect the actual level of emissions in the exhaust gas. To emphasize the point, the EGO sensors can not differentiate a gas mixture composed of 0 ppm HC at lambda=1 from a second gas mixture composed of 3000 ppm HC at lambda=1.
In addition to the primary functions of the EGO sensors for engine fueling control, comparison of the signals generated by the upstream and downstream sensor have been used to determine the oxygen storage capacity (OSC) of a three-way catalyst (TWC). If the engine is operating at an AFR cycling around lambda=1, and when a catalyst is fresh and OSC is fully functional, the front EGO sensor rapidly indicates the changing AFR. The downstream sensor signal, however, only periodically changes to indicate the changing AFR owning to the ability of the TWC to store oxygen during lean conditions and release oxygen to combust excess reductants (HC, CO, H.sub.2) during rich conditions. As the TWC ages and OSC degrades, the signal generated from the downstream EGO begins to more closely resemble the upstream EGO. In the extreme case of complete loss of OSC functionality, the downstream EGO signal directly reflects the upstream EGO signal. Those skilled in the art realize that the above description is general, but establishes the basic principle behind two EGO sensors to determine OSC properties of a TWC.
Vehicle manufactures (OEMs) have successfully established correlations between OSC and the EC conversion efficiency of TWCs for vehicles with less stringent emissions requirements. However, for LEV (low emission vehicles) and ULEV (ultra-low emission vehicles) applications, the correlations are significantly more difficult to establish and are inherently unreliable. As a result, large error is associated with diagnosis and the OEMs must turn on a MIL often well before actual emissions are exceeded. Premature indication of failure is required owing to the large error in OBD to ensure that the MIL will be illuminated for all true failures. It is highly desirable to improve the catalytic converter OBD diagnostic resolution to avoid extreme premature indication while still complying with regulations.
The prior art has recognized the deficiencies in the oxygen sensing systems. Recent patents have correlated temperature changes in the catalytic converter to a catalytic converter efficiency measurement thus avoiding the usage of EGO sensors. For example, U.S. Pat. No. 5,675,967 to Ries-Mueller correlates the time it takes the catalytic converter to reach operating temperature to an efficiency and U.S. Pat. No. 5,706,652 to Sultan takes temperature measurements before and after the catalytic converter.
A viable approach, made possible by this invention, which does not rely on OSC to determine compliance with emission regulations is the use of a calorimetric sensor to determine the concentration of the regulated emissions. Reference can be had to U.S. Pat. No. 5,707,148 to Visser et al.; U.S. Pat. No. 5,431,012 to Zanini-Fischer; and, U.S. Pat. No. 5,505,837 to Friese et al., incorporated by reference herein, for calorimetric sensors specifically designed for use in automotive, exhaust gas monitoring applications. Reference can be had to U.S. Pat. No. 5,177,464 to Hamburg; U.S. Pat. No. 5,408,215 to Hamburg; and, U.S. Pat. No. 5,444,974 to Beck et al. for monitoring system applications using calorimetric sensors. In Beck, the vehicle is operated at steady state condition with a lean air/fuel ratio whereat signals from the sensor are collected, histogrammed and analyzed to arrive at a pass/fail ratio. In the Hamburg references, a sample of the gas stream is tapped and sent to a chamber where the gas sample can be analyzed vis-a-vis the calorimetric sensor. Before this invention, the prior art used the calorimetric sensor only to monitor the exhaust gases and the monitoring was not done during the full range of engine operation.
The parent invention overcame the deficiencies in the calorimetric sensor systems of the prior art.
One of the basic problems solved by the parent invention arises from the very low concentrations of noxious emissions now being regulated. The low level of emissions cause very minor energy signals. If the calorimetric sensor is used to continuously monitor the exhaust stream as in Beck or as suggested in the prior art calorimetric sensor patents, the system energy and more importantly, the variations in system energy, simply prevent an absolute measurement from being recorded by the calorimetric sensor. The small energy signals attributed to the presence of minor concentrations of gaseous emissions cannot be consistently distinguished from any of the other system energy fluxes which produce significant quantities of energy affecting the sensor energy signal. A breakthrough in the emission monitoring art was achieved in the invention disclosed in parent application, Ser. No. 09/019,085, filed Feb. 6, 1998, incorporated by reference herein, which cycles the operating conditions of the engine to produce corresponding cyclical calorimetric signals which are detrended at the limits of the cycle and subtracted from one another to produce a relative signal indicative of the energy content of combustible gases in the exhaust stream. By selecting which engine operating condition is cycled, concentrations of selective emissions are detected and comparison of the signals obtained at the steady state limits of the cycle produce a relative signal which does not have to be correlated to a "zero" reference point to obtain an absolute measure of the concentration of the selected emission. The parent invention thus provides an accurate system for directly monitoring the catalytic converter efficiency based upon the emissions actually produced (and not some expected correlation based on an indirect property) at the very low ppm levels now being legislated which cannot be measured with current systems. The parent invention functions as an emission monitoring/catalytic evaluation system only. This invention extends some of the concepts discussed in the parent patent to not only monitor the emissions but also control the combustion process of the engine by a different system and method from that disclosed in the parent invention.
Within the vehicular art, frequency analysis has been known to accomplish OBD of engine components. In U.S. Pat. No. 5,493,902 and U.S. Pat. No. 5,499,538 to Glidewell et al. pressure waves are utilized to determine if the fuel pump is functional. In U.S. Pat. No. 5,725,425 to Rump et al. a frequency analysis is performed by a heat sensor to determine the presence of pollutants in ambient air and control the ventilation mode of the vehicle. A spectrum analysis of flow temperature is disclosed in U.S. Pat. No. 5,257,496 to Fattori et al. for turbine engines to control the air to fuel ratio. The spectral analysis detects the presence of oxygen compounds.
Outside the vehicular art, U.S. Pat. No. 5,244,775 to Reading et al. and U.S. Pat. No. 5,439,291 to Reading utilize frequency measurement resulting from the application of heat to determine phase transition of materials. U.S. Pat. No. 5,261,411 to Hughes utilizes frequency measurements to monitor and control the temperatures of blood within the heart for surgical procedures.
Despite the wide application of sophisticated techniques utilizing heat, there are many industrial processes and applications wherein chemical transformations can be utilized for monitoring or controlling the process if the chemical transformation can be induced, detected and analyzed.